Before OpenVPN there was IP-in-IP encapsulation, which is as simple as a VPN can get (no encryption, but that hadn't been invented yet). Wireguard is basically IP-in-UDP plus encryption. You know what else is basically IP-in-UDP plus encryption? IPsec, and it really sucks, for the same reasons OpenVPN does.

By "encryption, but that hadn't been invented yet" you mean like, specifically encrypted network streams, right?